You have basically three options: Promise, whose RAID card is actually software-based, ArcoIDE, whose card is RAID-1 only but entirely hardware-based and transparent to the OS, and 3Ware, whose cards are hardware-based but not transparent to the OS.
Promise's cards don't have a very good reputation in regard to their behavior under Linux. The drivers aren't very mature and many people have reported disk corruption and similar problems. Also, because their functions are entirely software-based, they tend to put more of a load on the CPU. They're quite inexpensive, however.
ArcoIDE's cards aren't very widely-used, but they seem to be quite reliable. They do only RAID-1 (mirroring), not RAID-0 (striping), so they're only useful for creating redundancy, not improving performance. Since they're entirely hardware-based, though, they don't tax the CPU at all, and the OS doesn't know that there's even a RAID card present. ArcoIDE's cards are quite expensive.
3Ware's cards are sort of the IDE version of SCSI RAID cards. They're hardware-based (the controller is on the card), but they do require that the OS has drivers for them. Their Linux drivers are quite mature and 3Ware has been very good in supporting Linux OSes. Since they're hardware-based they don't tax the CPU much, and the Linux kernel actually sees the IDE RAID array as a single SCSI disk. 3Ware's cards are the most robust of the three, and they support RAID 0, 1, 5, and 10 configurations and cards are available which can control 2, 4, or 8 disks. 3Ware cards are less expensive than ArcoIDE cards, but more expensive than Promise cards.
I've used all three, but I've used 3Ware's cards most extensively. I've been very pleased with the performance and reliability of 3Ware's 7xxx series, although the older (and no longer produced) 6xxx series had some problems. 3Ware's cards are as close to a SCSI-quality RAID card as you're going to get.
I haven't used ArcoIDE's new UDMA/66 cards, so I can't speak on the performance of their current line, but the performance of their UDMA/33 cards was on par with that of a single un-RAID'ed disk. Since the goal of RAID-1 is to create redundancy and not improve performance, though, this is quite acceptable. If you're just looking for redundancy and you don't want to have to fiddle with the OS, these are a good choice.
I'd recommend against Promise's cards unless you're strapped for cash, as they really seem to have quite a number of unresolved problems with their Linux drivers. They're also entirely software-based, so you don't really gain anything over doing non-hardware-assisted software-based RAID, and Linux's software-based RAID support is quite mature.
They do a really good job of reviewing hardware and giving you the dirt on stuff.
Tom's Hardware was easily once the best hardware review site out there. However, the good Dr. Thomas Pabst seems to have left the site to a gang of fools. Most of their stuff is still good, but it's no longer the best.
www.storagereview.com likely has some good IDE RAID card reviews. I would recommend a High Point card, as High Point controllers typically outperform Promise controllers.
amaroq - you missed the Adaptec 2400A, which is a pretty damn fine card, as are the 3Ware cards, we use both and haven't had any problems. It's worth noting that the Adaptec has a faster write speed in RAID5 then the 3Ware cards according to the benchmarks on StorageReview.