Let's see, aside from the obvious ones (Furlan & Dae Kim) a quick search turns up: -
Fredric Lehne; The ranger who delivered Sinclair's message in 'The Coming of Shadows' also played the US Marshal that nicked Kate and as I recall the "yellow eyed demon" on Supernatural.
William Sanderson; Deuce in Grail & Thirdspace played Oldham on Lost. Of course better known as J.F. Sebastian from Blade Runner.
Julie Ow: Played a MedTech in 'Rising Star' (the one that briefed Franklin on Marcus & Ivanova if memory serves) and a by a bizarre coincidence (or typecasting), a Nurse on Lost.
Julian Barnes: Played Lord Durano in 'Into the Fire' (the one that told Londo who really had Adira killed) and on Lost he played a Dr. Woodruff in 'Par Avion'.
I also turned up a bunch of voice actors who dubbed for the non-english versions of both shows, including one that did the French dub for Mira on both occasions.
Behind the camera there's also Mitch Suskin who was VFX supervisor on B5 & Lost.
Another interesting little titbit I came across: some of the Dharma station logos (see
here) incorporate logos from B5! I can spot the Vorlon Empire, Minbari Federation & one of the Warrior Caste clan emblems (wind swords or star riders, I think.) Methinks Suskin may be behind these.
[EDIT]
Looks like those emblems are actually fakes. Serves me right for not paying attention!