Magicka. Magicka is awesome. And you can get it for like 4 quid. If you're really tight for money, it's £3.50 on Gamer's Gate. This game really fills me with joy - it's funny, it's interesting, it's funny, it has the best magic system of any game I've played (in fact, it's the magic system I've always wanted to see in a game), it has really amusing DLC that I want to get not because I want to own them but because they are just sound that awesome. Also, it's funny.
The Walking Dead and XCOM are 50% off on GameFly [http://www.escapistmagazine.com/forums/read/9.396858-The-Walking-Dead-and-XCOM-50-off-on-GameFly] - never played them but they were my top most wanted games for a while and I was waiting for the Steam Christmas sale. GF just made me an offer I couldn't refuse.
Cthulhu Saves the World + Breath of Death VII - if you're into some old school-ish JRPGs - you'll probably like them. Heck, I've never played one of those and I was still very satisfied. Oh, the two pack less than a pound - at £0.66 the games are legitimately cheaper than chips. Although I think they are also on consoles, so you might not really need them on a PC.
If you like turn based strategies - The Heroes Pack on Steam is really good - it has Heroes of Might and Magic 5 plus the two expansions and it's a very good game. As a bonus, you get Dark Messiah, which, while I'll not call a very good game, is a fine experience and if you're buying Heroes, you shouldn't pass up that offer.
Speaking of the whole Heroes thing, well if you don't mind older stuff, GOG recently had a sale of the Might and Magic pack. The sale ended, but I am almost positive the pack would come back[footnote]They had several packs on sale at the same time, now they have one at a time every day, so the M&M one would probably show up again[/footnote] - it was an amazing value - 25$ for Heroes of M&M 1 to 5, Heroes Chronicles, Might and Magic 1 to 9 8, and Crusaders of M&M. Yeah, all this for 25$ - everything was 75% off, if you chose individual titles, you'd only be getting them 50% off (actually, that's also the price break at the moment) - when I checked to see how much would my preferred games (HoMM 3, Chronicles, HoMM 5, M&M 7 and 8) cost it was 25$ as well. The pack is just too good to pass up, a shame I have most of the games in it. It doesn't have Dark Messiah, however, which is somewhat a shame but it's DRM free on the other hand and filled with other amazing games.
So, continuing on GOG bundles - another thing that may show up again on sale is the Bullfrog bundle. It's 12$ for Dungeon Keeper 1 and 2, Syndicate, Populus, Theme Hospital, and Magic Carpet. Since I cannot express my thoughts on this thing myself, I summoned a spirit of the dead better gifted in the ways of speech than me:
HI, BILLY MAYS HERE. THIS AMAZING BUNDLE INCLUDES CLASSICS SUCH AS DUNGEON KEEPER 1 AND 2 - GAMES YOU KNOW ARE WORTH TWELVE DOLLARS BY THEMSELVES, BUT YOU ALSO GET OTHER AMAZING GAMES - SYNDICATE, POPULUS, THEME HOSPITAL, EVEN MAGIC CARPET! ALL THIS FOR THE LOW, LOW PRICE OF TWELVE DOLLARS.
Thank you, Billy Mays. You know what, you convinced me to buy the pack! Also, I just noticed I don't have DK on GOG yet, so what better way to fix that by getting all the other games that are on my wishlist, too. Well, aside from Populus, just not my thing, and I hadn't heard of Magic Carpet before.
Then, there is Heroes 6. Now, if you've never played Heroes before...you may get a good or bad impression. But I suggest you download and try the demo, see if it interests you. It deviates slightly from the older games, but I myself like it, I understand it may not be everyone's cup of tea, though. It's £10 on Green Man Gaming. Actually, this requires a section on it's own, so look for it below.
Moving on. Frozen Synapse is an amazing tactical shooter. Really smooth and replayable. Sweet soundtrack, too. I'm loving it. For some reason, if you get FS off Gamer's Gate individually (FS, the DLC, and the soundtrack), you'll pay less than the full pack, as the individual items are 75% off, while the full pack with all of them is 50% off.
Torchligh 2 is a very fun Hack & Slash dungeon crawler game. I'm loving it. It's 50% off at GameFly.
Then, there are the ES series - Morrowind, Oblivion, and Skyrim. For Skyrim check below under Green Man Gaming. These games are quite awesome on PC, as you can stuff 'em full of mods. Oblivion, and especially Skyrim, can look really amazing modded. Actually, Morrowind, too, although not as much as those two. Also, while not really as moddable, and quite...dated, I suppose, Daggerfall and Arena are free from the official TES website.
Last but certainly not least, are my two favourite games. Well, one is technically series - Legacy of Kain, the other is just a game - Bloodlines. For Bloodlines...for some reason they sell it for too much, IMO, wait until it's 75% off, maybe at least 50%. For LoK, I suggest GOG - it's cheaper than Steam and there was a LoK pack that might come back.
[HEADING=2]Green Man Gaming[/HEADING]
First of all, you can use this code to get 30% off any PC titles: GMG30-DPLIM-DN831 - you can apply it once per purchase, so if you want to buy several games, do it one by one. So, to recap - Heroes 6 is £10, but you can get it for £7 with it. Batman Arkham Asylym GOTY is also on sale, I got it for £1.74 a couple of hours ago. I'm pretty sure it's worth that money. Arkham City GOTY is £5 before the code. Deus Ex (the first one) is £2 pre-code, too. Disciples 2 Gold is £5 and it's another good turn based strategy, similar to Heroes. Hitman Blood Money is £4 - it's my favourite Hitman game (still haven't played Absolution, though) - it's actually cheaper than Hitman 2, and the same price as Hitman 1. Dunno why they don't have 3 (Contracts) on there - I've not seen it anywhere, actually - only GameFly lists it as not for sale. Skyrim is reduced price, and if you apply the code, you'll get it at a total of 65% off, for about £16. You have to get that game on a PC and stuff it with mods. It's the law. Well, unless you're not really into TES series...but do think about it.
Finally, since you have DMC4, I'm pretty sure you'll like DMC3. Unless you have it on a console, I suppose. But do play that game on any platform.