Stompy-bot management sim BattleTech may not technically have native mod support, but developers Harebrained Schemes aren't so proud as to keep all their data under lock and key. Realising that folks would want to poke around inside the guts of their latest game, they left much of its file structure open and human-editable.
One of the nicer little mods to come from this is Unlock Full Map After Obtaining The Argo, which is a very boring name but does just what it says on the tin. Ideal for the mercenary captain that would rather leave the main story behind and go freelance for a while.
The Unlock Full Map mod still requires you to work your way through the tutorial and the first few campaign missions, but once you've access to your own personal ship, the sky's the limit. All the systems that would normally be locked off until late or post-story will be available, allowing you to wander the galaxy, picking and choosing contracts at will in Davion, Canopan, Liao and Marik-held space.
As BattleTech isn't really designed for mods, you'll need a couple prerequisites before you download your map-unlocker, most notably the ModTek modding framework, and the BattleTechModLoader tool to handle the heavy lifting so you're not fiddling around with a dozen different directories worth of data files. These are required for just about any current BattleTech mod.
Outside of this campaign-broadening mod, there's some interesting stuff being put together by the community. One to keep an eye on (although be wary of its affect on balance) is the Lost Weapons pack, adding in a couple rare new tools of destruction to the loot tables including stability-wrecking Rotary Autocannons in both AC/2 and AC/5 calibres, plus a more powerful (but heavier and hotter-running) LRM variant that you can find in former Star League systems.
That's exactly the kind of stuff I'd like to see mods add to the game - little bits here and there to flesh out the BattleTech galaxy a little more, with each part of space having its own preferred combat styles. There are plenty more mods, including some game-wide balance overhauls, but you're probably best off keeping your tweaking and tuning relatively minor until you've finished one run through the game. Still, you can find plenty more over on the BattleTech Nexus here.
Props to PC Gamer for spotting this particular mod.