Mac OS X only: It looks like the folks carrying on Quicksilver's legacy aren't just messing around — they've released yet another update to the formerly abandoned app launcher. The B58 build focuses on stability, especially concerning issues for those running Snow Leopard, as well as making sure older plugins are compatible. There are also a few UI tweaks and bug fixes, as well as increased performance.
If the last build (B57) was suitable for you, you should find this version even better. Users who had trouble with the last build, though, finding that it actually decreased performance, will find that this build is better than B57, but still not as snappy as B54. However, if you've stuck with B54 and itching to get that preferences window working again (something that stopped working with the release of Snow Leopard), you may find the newest build a fine compromise — it's not quite as fast, but it is far less buggy as far as customising plugins go. Since Quicksilver seems to be acting differently for everyone, let us know how the upgrade goes for you in the comments.
Quicksilver is a free download, Mac only.