Among engine testing sites the most famous are CCRL (Computer Chess Rating Lists) and CEGT (Chess Engines Grand Tournament). My personal preference is the CCLRs' 40/40 list because they include commercial, open source, private and free engines and you can easily see how each engine has scored against any other engine. All engines are tested with ponder off, a general opening book of up to 12 moves and 3-4-5 piece EGTBs. And, the engines are tested on an Athlon 64 X2 4600+ (2.4 GHz) with a time control of 40 moves in 40 minutes. That makes the results a little more reliable than blitz time controls or testing that's done on monster computers using a million cores.
Of course these days Stockfish is the engine to beat, but other engines have certain characteristics that make them useful in some positions and whether or not you are preparing for over the board play or correspondence play.
However, if you are looking for weaker engine that you can practice against then I recommend Cinnamon. Last year I posted on this engine (you can play against it on the left). I like to play against it because its low-2000 rating makes it challenging, but not unbeatable and it doesn't make one stupid blunder then play like a GM for the rest of the game.
Many older engines were similar. For example, when Chessmaster was playing about 1600 or so, I realized a few of my postal opponents were using it, but because of its low rating it wasn't really a problem.
Back in the old days (before computers) at Chess Review I had started out in Class C (about 1500) and worked my way up to over 2000 and had qualified for several of their Golden Knight Finals. It took a lot of effort. A large collection of opening books, careful analysis and the keeping of copious notes were the keys. Those were fun events because in those days quite a few OTB masters played postal chess and I got to play a few of them.
Then one day when Chessmaster crossed the master threshold and I was playing about a dozen postal games with the USCF which had taken over from Chess Review and something happened.
One of my opponents whom I knew personally from a few years before had never been rated more than 1500, but when I got my assignment his rating was around 2000. After 20 moves or so, I got suspicious and checked his moves with Chessmaster and discovered that's what he was using.
Curious, I began checking my other games and discovered at least half of my opponent's were using Chessmaster. Disgusted, even though it cost me 300 rating points, I resigned all my postal games and abandoned correspondence play for 12-13 years. When I came back, it was on internet sites and things had only gotten worse.
For serious analysis you need a couple of different strong engines and they have made us all armchair Grandmasters, but if you're into over the board play and want to practice, strong engines don't play like people do. Instead they play like a GM, blunder, then go back to playing like a GM. The solution is to play against some of the older engines that aren't master strength. That's why I like Cinnamon.
If you're looking for some specific older engine or are just interested in fiddling around with them, here are some sites you might like to check out.
ChessMosaic - The author says that you can search, download and explore over 1.4 million games from his collection, but I noticed that it was current only to 2012. You can view and download his collection of chess engines; there looks to be over 250 of them. The engines are listed in alphabetical order by whether they are Winboard engines or UCI engines, so finding the one you want is pretty easy.
ChessBites - Over five million searchable games, opening novelties, mates, ending, more
Ridderkerk is an old (up to 2013) site that has a lot of interesting downloads.
Rebel – This is a summary of REBEL products dating back to the 1980s. Download of a lot of engines is available.