I'll add more when I think of them, or when I bump into them
One word of advice:
Even when a third party programm proves not to be bannable, you can never know for sure. These are NOT from Steam itself so in a way you will ALWAYS risk a VAC ban. Just make sure you're not running any when playing a VAC protected game!
Steam Service Status
See if Steam is up, and how the servers are.
Steam Achievements
When going for Steam Achievements, these might come in handy.
Astats
On this website you can find ANY Steam game with Achievements, and ANY Achievements ever known to Steam. Even if they have been long since removed, Astats knows it all. If you make an account here you can keep track of what your achievements are and how far you're done with the game according the Achievements you have. You can even see when you finished the game, and how long it took you.
If you don't really understand, take a look at mine.
HowLongToBeat
Which brings us at howlongtobeat. This is a website where you can also track your Achievements, BUT... Not just Steam achievements. ANY game you've EVER played can be found here. The only downside is, you have to fill it all in yourself. There's no way you can link or sync to your Steam, PSN, xbox or whatever. Of course you may have forgotten how long it takes to play through a game you've played a loooong time ago... But you don't have to fill in everything. If you don't remember, they just take the average time of completion and put it on your profile. You can always edit the entries you've made.
Steam Achievement Manager
Steam Achievement Manager, or SAM for short, is a third party program where you can unlock your Achievements on Steam without even having to do anything. You run the program, pick a game, tick on which achievements you want and store them. Boom achievements earned.
Obviously this is cheating and even having this on your computer will risk you a VAC ban. But hey, we all know people who can't resist right? Just use it wisely then. There's nobody to blame but yourself when you get caught.
Steam Games
If you're more of a person that is on the hunt for cheap(er) games, discounts and sorts like that, these might come in handy.
Enhanced Steam
This is a browser add-on which allows you to find more sorting options in the Steam Store, as well as your Wishlist. You can now sort on discounts. When visiting a Steam store page, it will show you where it is currently cheaper, as well as the moment that game or DLC was at its cheapest (and where).
Please note: Even though Enhanced Steam shows you lower prices for games and DLC, this doesn't mean there aren't any lower prices for that game or DLC at the time. It doesn't see ALL websites selling keys, obviously. So you might want to dig even deeper if you really want the cheapest key(s).
Library Manager
It doesn't have anything to do with purchasing games, but this is a useful program when you want to make space on your harddrive without having to uninstall and install it again. Takes way too much time right? This program moves it for you without too much trouble.
Steam DB
Steam DataBase shows you more information on Steam store games and DLC. You can also find all the discounts that are currently going on at Steam itself, for how long, and how much longer they last. This is perfect when you're looking for a certain game and you want the lowest price. Games on your wishlist are highlighted, so easy to find.
Steam Trading Cards
If you're interested in collecting Steam trading cards, these might come in handy.
Idle Master
Idle Master is really just a program that runs your games for you, for the card drops. That's about all it does. Steam will see you playing the game, while you're not really playing it. Even though this is a third party programm, its not a bannable offense... Since well lets be honest, you COULD do this yourself without a programm as well. There's no way of checking it and hence why Steam doesn't take action on this. At least, not any action known.
Steam Inventory Helper
The Steam inventory helper, or short SIH, is a browser add-on that will show you prices of games and cards when you visit a page. If you look around closely in the settings, you can also enable quick buy and quick sell. This will automatically place your cards on the Steam community market, or you can buy directly from there. Without all the extra steps like conformation. Its quite useful sometimes.
Please note: Even though SIH shows you lower prices for games and DLC, this doesn't mean there aren't any lower prices for that game or DLC at the time. It doesn't see ALL websites selling keys, obviously. So you might want to dig even deeper if you really want the cheapest key(s).
Steam Tools
Here you can find various tools that help you determine the worth of Steam items. Think of cards, but also backgrounds and emoticons. You can find the price of complete card sets here, including the ones you already own. This is most useful when looking at boosters, instead of cards.
SteamTrade Matcher
This is a very useful website to find other people with cards you still need, and they need the ones you have as duplicates. This is also a good way to find Steam trading bots (not from Steam itself, but people who use scripts for them) - since they have loads more cards than regular users. At least they do most of the time.
Personally I prefer trading to users before bots, because users might actually WANT that card you may have for them, bots don't care. On the other side, bots do trade way faster, and you won't have any delayed trades.
Useful Steam bot list
A little list with Steam bots I often trade with to get my Steam cards.
They respond fast, and fair. Some even trade cross set.
- birdman 1:1 card trader has a large pool of cards.
- Crushed Bomb | 1:1 Trading Bot is one of my favorites, has a large pool of cards.
- GFG Trading Bot one of my favorites, has a large pool of cards and does cross set too.
- N!vq | 1:1 Trading Bot
- Vanilla「バニラ」 isn't always as fast but its still a bot.