Alright, this one is going out to all acid-freaks here

I bet you always wanted to have a Roland TR-909 rhythm composer accompanied by the legendary TB-303 bassline. I've been looking for the perfect combo but had no success - till now
I found two decent VSTi plugins that can bring back the feeling of oldschool music production, they especially come in handy when creating acid stuff

They aren't the newest stuff, but definitely do the trick.
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D-16 Group Drumazon
http://www.d16.pl/index.php?menu=14
Drumazon is an excellent representation of the famous Roland TR-909 groovebox. It gives 99.9% the same sound, it's outputs can be assigned differently to effect channels, and it can be programmed in the same way as the original hardware. It comes with a small extra called "Random Mode", which can create random patterns if you feel like it. Awesome stuff, pure genious!
AudioRealism BassLine 2
http://www.audiorealism.se/abl/abl2_announcement.htm
One of the hardest instruments to emulate on a PC is definitely the Roland TB-303 bassline synth. Few VSTi's had nearly the same sound, but they usually bleed out when it comes to maximum ENV + RES knob values. BassLine2 is the best I ever encountered with. It's still not perfect, but for techtrance, acid and freeform it is the ultimate solution. It can be programmed the same way as the classic 303, and with FX + MIDI remote controlling, it gives almost the same feeling

Masterpiece!
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Here's a small demo I wrote using these two babies:
http://www.dezigned4ravers.com/drumazon-abl2_demo_by_dj_flatline.mp3
I think it's convincing enough