Sonycowboy likely has totals for all the Sega games they haven't published and the GBA stuff too. There's lots of Sega products other people handled though, off the top of my head...
Acclaim
-18 Wheeler: American Pro Trucker (GC/PS2)
-Crazy Taxi (GC/PS2)
-Ecco the Dolphin: Defender of the Future (PS2)
-Headhunter (PS2)
-Worms 3D (GC/PS2/XB)
-Worms Forts: Under Seige (PS2/XB)
Atlus
-Shining Force: Resurrection of the Dark Dragon (GBA)
-Shining Soul (GBA)
-Shining Soul II (GBA)
Microsoft
-Ourun 2 (XB) *soon*
-Phantasy Star Online Epidoe I&II (XB)
-Shenmue II (XB)
Namco
-Vampire Night (PS2)
Nintendo
-F-Zero GX (GC)
Nokia
-Alien Front (NGE)
-Pocket Kingdom: Own the World (NGE)
-Puyo Pop (NGE)
-Sega Rally Championship (NGE)
-Sonic N (NGE)
-Super Monkey Ball (NGE)
-Virtua Cop (NGE)
SNK
-Puyo Pop (NGPC)
-Sonic the Hedgehog Pocket Adventure (NGPC)
THQ
-Altered Beast: Guardian of the Realms (GBA)
-Columns Crown (GBA)
-Crazy Taxi: Catch a Ride (GBA)
-Jet Grind Radio (GBA)
-Phantasy Star Collection (GBA)
-Pinball of the Dead (GBA)
-Puyo Pop (GBA)
-Revenge of Shinobi (GBA)
-Sega Arcade Gallery (GBA)
-Sega Smash Pack (GBA)
-Sonic Advance (GBA)
-Sonic Advance 2 (GBA)
-Sonic Advance 3 (GBA)
-Sonic Battle (GBA)
-Space Channel 5: Ulala's Cosmic Attack (GBA)
-Super Monkey Ball Jr (GBA)
-Virtua Tennis: Sega Professional Tennis (GBA)
...plus there's 4 Sega GBA games they self published...
-Astro Boy: Omega Factor
-Chu Chu Rocket!
-Sega Rally Championship
-Sonic Pinball Party