OPL3-SA (Yamaha YMF71x) based cards - Genuine OPL3, good SBPro compatibility, so so software wavetable under Windows (3.1 even), decent sound (if the card itself is good)
OPTi 82C929 based cards (one with a discrete genuine OPL3) - OPL3, good SBPro compatibility (reversed stereo), mine has very clean sound
Yamaha YMF724-based cards - decent DOS compatibility for a PCI card, hardware-assisted XG MIDI (better than the ISA cards AFAIK), true OPL3

There are whole families of cards based on C-Media chipsets, like those from HT Omega, Sondigo, Auzentech, Asus, and Razer. The same goes for the VIA Envy24, with cards from Onkyo, M-Audio, Terratec, and Chaintech. They would be alternatives mostly to SB Live and SB Audigy (some are still made today, so could be considered alternatives to X-Fi and SoundCore as well).

I'm enjoying my Turtle Beach Santa Cruz. Probably the best contender to the Aureal Vortex2 cards.

AZT-2316/A/R based sound cards.
Similar to the AZT-2320 based cards, with the exception that it is not Plug 'n Play.
For DOS, this works better for me since you then don't need to load a Plug 'n Play configuration manager.
Supports Sound Blaster, Sound Blaster Pro II and Windows Sound System.
Has a Wave Blaster header and MPU-401 UART MIDI interface.

First generation Sound Galaxy sound cards (NX-Pro, Basic16/Pro16, etc.).
Supports the same sound standards as above, including Disney Sound Source and Covox Speech Thing.
Doesn't unfortunately have an MPU-401 UART MIDI interface (very similar to Sound Blaster Pro II in this respect) and sightly noisy.

Orchid SoundWave32. A great native WSS card with SB support. One of the strong points is very solid MT-32 emulation.

RacoonRider wrote:

Orchid SoundWave32. A great native WSS card with SB support. One of the strong points is very solid MT-32 emulation.

I've heard that this soundcard has very good MT-32 emulation but, how good is it?
Are we talking being able to interpret sysex messages as well?

I happen to have one of these but, with the standard ROM chips which sound horrible.
However, with the optional Invision ROM chips, it apparently sounds quite good - does anyone have an idea where I could find some of these Invision ROM chips?
Only downside is that it only supports Sound Blaster and not Sound Blaster Pro.

Another sound card that comes to mind is the Guillemot Maxi Sound Home Studio 2 - has the ESS1868 DAC (which also supports up to Sound Blaster Pro) and the SAM9407 MIDI synthesis. With the correct samples loaded, it sounds just like a Roland Sound Canvas.

Asus Xonar STX?
Not retro, but you since you listed X-Fi anyway.

ESS cards are cheap and cheerful. Crystal cards work well too but need a TSR.

The FortéMedia FM801 is PCI and has good Sound Blaster support despite having no SB-Link header. One of few cards where the FM Synth is useable in XP.

Labway Xwave 6000 is a good Yamaha based card. Haven't had the chance to fire mine up, yet.

Yamaha Audician 32 Plus is another good one. As alexanrs posted, Yamaha YMF71x cards are good, so I'll throw this one in. I've got one running in my super socket 7 system. Has SB Pro and WSS compatibility, and also has a real Yamaha OPL3 chip (YMF718-S). Also been reported to be pretty quiet.

I use a logitech soundman wave for sb emulation. It uses a opl4 chip. Sbpro compatibility is apperently a hit and miss.

Philips Seismic Edge / Acoustic Edge which use the VLSI Thunderbird 128 or Thunderbird Avenger (newer chip). Excellent 3D audio via QSound hardware implementation, including an excellent headphone mode reminiscent of Audigy/X-Fi. EAX support is well done, unlike with various other chips (Crystal and Aureal come to mind). Make sure you get the Thunderbird Avenger versions of the cards if you want to use them with Win 2000 and XP because the Tbird128 didn't receive fully functional NT5 drivers.

ESS AudioDrive 688 - has a discrete OPL3 chip
Yamaha YMF-71x - as already mentioned, has true OPL FM synth
A few Crystal-based cards, i.e. those based off the CS4232 chip, are pretty good as well.

Guillemot Maxi Sound Fortissimo

Maxi Sound Game Theater 64.
One of the last isa cards, for me it's better than an awe64.
4mb onboard + 16mb expanded by generic Edo ram
No Yamaha chip but the fm sounds from the ess chip is 99% equals an Yamaha one.
There's driver for Windows 3.11, 95, 98 and 2000.
5.1, full duplex.
May be a bit hard to find
Plug and play.
Can run directly into ms-dos with some special did drivers (that adjust eat the 640kb ram area)
There's some joined of mt32 emulation, 4d stunts and a really old ultima game worked, but without the special sound effects...

Thanks so much for all the responses. Feel free to keep them coming.

So far this sums up a few of the responses (omitted some that share chips with a few I listed, as well as some newer cards - I'll be looking at newer cards later on):

Crystal CS4630 based cards (Turtle Beach Santa Cruz)
Crystal CS423x based cards
Orchid SoundWave32
FortéMedia FM801
Philips Seismic Edge
ESS ES1868 based cards (Maxi Sound Game Theater 64)
Yamaha YMF744 based cards (Guillemot Maxi Sound Fortissimo)
Logitech Soundman Wave

I think I may have several of these cards - but that Orchid SoundWave32 piques my interest the most!

I'm going to try to update in the next couple days with a few other cards I have laying around and see what people think of them...

Can anyone tell me more about the following cards / chips? I'm especially curious about the first three.

Breve AIM16F
ALS200
Cardinal MPC700 (might have Analog Devices ECHO chip)
Crystal CS9236
Ensoniq SoundScape VIVO 90DB
ESS Maestro-1
MagicSound 16