Hi Jared - saw this at work (where I didn't have my lists).
I thought I'd attach what I took to Tumult - it seemed to work pretty well as a balanced/flexible list. It certainly was competitive with the lists I played.
As Dave said, though - pretty open question. Work out the style of things you would like to do and then try to put a list around that.
Anyway, list with notes.
2000 Pts - Codex: Space Marines Roster - Tumult 2011 List
Total Roster Cost: 2000
HQ: Space Marine Captain in Power Armour (1#, 145 pts)
1 Space Marine Captain in Power Armour, 145 pts = (base cost 100 + Storm Shield x1 15) + Relic Blade x1 30
A good, cheapish HQ choice. Tough to kill (storm shield), but able to take on pretty much anything (relic blade). I dislike Space Marines insofar as a power-fist to the face kills them instantly, so ... I felt the storm shield gave him a reasonable chance. (Thanks to Mike for pointing out the option - turned out to be a winner.

)
HQ: Space Marine Librarian in Power Armour (1#, 100 pts)
1 Space Marine Librarian in Power Armour, 100 pts
Cheap HQ with Null Zone (reroll successful enemy invuln saves); and Gate of Infinity (gives good strategic options).
Elite: Terminator Assault Squad (5#, 200 pts)
4 Terminator Assault Squad, 160 pts = 4 * 40
1 Sergeant, 40 pts
Face-smashing squad - bundle in the Captain and Libby, equipping all with TH/SS to make them tough. Would probably make a couple use LCs in future builds - the whole squad striking last always does suck somewhat.
Heavy Support: Land Raider Crusader (1#, 275 pts)
1 Land Raider Crusader, 275 pts = (base cost 250 + Extra Armor 15 + Multi-melta 10)
Terminator chariot. Anti-infantry fire with the MM to greet enemy vehicles.
Elite: Ironclad Dreadnought (1#, 160 pts)
1 Ironclad Dreadnought, 160 pts = (base cost 135 + Heavy Flamer 10) + Ironclad Assault Launchers 15
Cool/distractor unit. Scary enough to draw some fire. Can kick arse if gets into close combat (though often got shot before it did.)
Okay - the squads. Would combat-squad them where possible - the heavy in a stay-behind squad which would sit back and snipe; meltagun-or-flamer/power-fist squad would go in the rhinos and move forward to assault.
Troops: Tactical Squad (11#, 230 pts)
9 Tactical Squad, 147 pts = 9 * 16 (base cost 16) + Sergeant 51
1 Sergeant, 48 pts = (base cost 23) + Power Fist x1 25
1 Rhino, 35 pts
Troops: Tactical Squad (11#, 235 pts)
9 Tactical Squad, 152 pts = 9 * 16 (base cost 16) + Meltagun 5 + Sergeant 51
1 Sergeant, 48 pts = (base cost 23) + Power Fist x1 25
1 Rhino, 35 pts
Troops: Tactical Squad (11#, 245 pts)
9 Tactical Squad, 162 pts = 9 * 16 (base cost 16) + Meltagun 5 + Lascannon 10 + Sergeant 51
1 Sergeant, 48 pts = (base cost 23) + Power Fist x1 25
1 Rhino, 35 pts
Troops: Tactical Squad (10#, 210 pts)
9 Tactical Squad, 162 pts = 9 * 16 (base cost 16) + Meltagun 5 + Lascannon 10 + Sergeant 51
1 Sergeant, 48 pts = (base cost 23) + Power Fist x1 25
Heavy Support: Predator (1#, 100 pts)
1 Predator, 100 pts = (base cost 60 + Extra Armor 15 + Heavy Bolter (each side) 25)
Pillbox. Surprisingly effective - always seemed to survive given it was considered less of a threat than Ironclad or the Crusader. Still - would sit back with the autocannon and heavy bolters and either nail skimmers (DEs again) or slowly strip wounds off squads.
Heavy Support: Thunderfire Cannon (1#, 100 pts)
1 Thunderfire Cannon, 100 pts
1 Techmarine Gunner, 0 pts
Awesome unit for 100 pts. Fragile (dies on any glancing/penetrating hit), but in more than half of the games, was able to remove opponent 'death' units wholesale with the no-cover round. DEs hate Thunderfire cannons; actually - any non-marine unit hates them given long range and either S6 round or S5, no-cover round. Haven't used the difficult-terrain round to date.
Anyway - hope this helps.
Cya,
Andrew.