I use the demo version of Rybka, 2.2n: http://www.rybkachess.com/index.php?auswahl=Demo+version
I also use RobboLito, which is new, GPL, and currently the strongest program out there. However, there are rumours that it was developed from stolen Rybka 4 source -- I don't know whether this is true. Also it doesn't use multiple CPUs, and it can only show one line (Scid can let Rybka show the best x moves, but it doesn't work with RobboLito). http://www.chesslogik.com/robbolito.htm
I've read a bit about Robbolito, Ivanhoe and Firebird (which can all be downloaded from Chesslogik, here), and I'm currently investigating which one to go for. I've not updated my engines yet -which is the reason for asking the question in the first place.
Scarblac - do you play against either Rybka or Robbolito, or do you only use them for analysis?
It is my 4th day with SCID. I've just plugged in stockfish.
I don't know whether it is "as good as --or-- better than" the other engines though. Any thoughts?
I use Chessbomb to watch live matches, and they use Stockfish there to analyse the games live. It seems pretty good to me. It's free, which is a bonus! Let us know how you get on with it.
Also, could you tell us whether you use it for analysis only, or do you play against it on SCID?
Russell: LOL, I've been at it for only 4 days, an hour here and an hour there. So I am by no means a power-user. At least, not yet.
Yes, I too came across stockfish at chessbomb during the WCC2010. Though I know that v1.7.1 is available, I chose to go with my Kubuntu repository which has only v1.6.
The only comments I can make about it right now are:
). Since then, I've been carefully watching the CPU temperatures.
It seems that can go upto 18-ply depth without overheating (taking ~300seconds for that, in most middlegame positions) on a Dell Inspiron 1525. I should be able to go deeper, but SCID quirks make it difficult. e.g., when I type stuff in my repertoire editor, the key strokes -- esp., the back arrows -- interrupt the game board and restart the engine analysis.Sure, I'll keep you guys posted on how it goes for me. 
Just a quick question: which free engines do you run with SCID? (If they're not Scidlet and Crafty, ie., the engines that come with SCID, please give the names & website links.)