MonoKey is a free VST Module Architecture 1.0 MIDI plugin that emulates fingered portamento for synths.
Many older synthesizers (for example Roland's Alpha Juno-1/2) only offers a portamento switch to turn portamento on or off. When it is on, all notes will have portamento, even single notes. MonoKey will enable fingered portamento, i.e. portamento will only be active for notes that overlap. It does so by selectively sending standard MIDI portamento on/off controller messages. For this to work effectively, your synth should be in monophonic or unison mode.
NOTE: VST Module Architecture is only currently supported in Steinberg's Cubase SX and Nuendo. Just drop the .dll file in the sequencer's Components folder.