If you didn´t know already, Audiotool is a free powerful online music production studio right in your browser.
Vinyl is a plug-in that lets you simulate the dust, scratches, and warp of a worn record and the electrical and mechanical noise of the turntable it’s on. Free from Izotope (Needs e-mail registration).
PROCESSORZ° is a cool combination of 25 colorful effects which can be played in real time as an instruments via a Midi keyboard. Download from the NI Reaktor User Library.