Dr Julian Bashir (DS9) crops up in a two-part ST TNG episode
Robert Picardo (Voyager) reprises his role as the emergency medical hologram in Star Trek: First Contact along with Etan Phillips (Neelix, Voyager) who confronts the Borg in the holodeck scene, and Dwight Schulz (The A Team) cameos as 'Mr Barclay'.
'Captain Sulu' appears in a Voyager episode & Scotty popped up in ST-TNG.
My personal favourite though, is Stephen Hawking as himself, playing cards on the holodeck with Einstein, Newton & Data. Nicely played geeky science gag
Interesting note that there is one person who has appeared in just about everything Trek in all it's guises. It's Majel Barrett, who was most famously known as Dr McCoy's nurse Chapel. However, a few years earlier she played 'No1', Enterprise First Officer in the original pilot episode with Jeff hunter as Captain Pike. she returned as Chapel in the ST movies as well as the oft-overlooked animated Trek series of the early 70's. She was also Deanna Troi's overbearing mother in ST-TNG and provided the computer voice in that show along with DS9 & Voyager. She also did the Enterprise computer in the 2009 film, shortly before her death. Leonard Nimoy comes a close second.