While Londo was his most visible role, Peter Jurasik is a character actor with over thirty years and 70 roles under his belt; for our purposes it's most interesting that he, like Boxleitner and David Warner, was in TRON (as CROM, an accounting program destined for the games grid).
Byron the telepath is also the Medic from Team Fortress 2, among many others.
When the computer is rebooted after a systems shutdown, it comes back online with an obnoxious, stubborn personality voiced by Harlan Ellison.
Ellison also provided the voice for Zooty's talking prop when comedians Rebo and Zooty visited the station.
Ardwright Chamberlain — Nicolai of Shadow Hearts: Covenant — is Kosh.
Mel Winkler, Aku Aku from the Crash Bandicoot series, appears in the episode "And The Rock Cried Out, No Hiding Place".
In the 2002 TV movie Legend Of The Rangers, Sarah Cantrell is played by Myriam Sirois — better known to a generation of North American anime fans as the voice of Akane Tendo from the English dub of Ranma 1/2.
Line To God: JMS was a frequent poster on the Usenet group rec.arts.tv.scifi.babylon5.moderated during the show's run.
Death of Personality: A form of capital punishment practiced by Earth Alliance, actual execution having been outlawed except in mutiny and treason cases. Also known as mindwipe, and first spoken of in "The Quality of Mercy".
True Companions: Most of the B5 core cast is still tight-knit to this day, particularly Claudia Christian, Bruce Boxleitner, and Mira Furlan. Furlan even says that Boxleitner is still their Captain.
Word of Gay: Well, Word Of Bi. For both Lochley (according to Neil Gaiman) and Cartagia (according to Wertham Krimmer). Although in both cases there was definite innuendo on-screen.
Hugo Award: The Hugo award the show won, for either "The Coming of Shadows" (1996) or "Severed Dreams" (1997) appeared in on Ivanova's desk in "Sleeping in Light"
Back when nearly all of the World Wide Web could be linked to on one page, it seemed that most web sites had some sort of sci-fi series tribute somewhere. Star Trek: The Next Generation was a particularly popular choice for enshrinement (it was on at the time, see?). B5 has one up on it, though: the NCSA Mosaic browser had a semi-secret page (about:b5) where the developers expressed their love for the series and provided a few links to fan sites in case you wanted to learn more.