IMHO, the most versatile commuter/trail bike you are going to find for <$300 is a 1990s rigid (no suspension) steel brand name MTB. If you watch for the chrome moly models you can get very durable frames that are still comfortable and not overly heavy. Here are a few possibilities:
Giant Boulder or Yukon
Specialized Hardrock or Stump Jumper
Trek 820 or 830 Antelope or Mountain Track (26") or 720 or 730 MultiTrack (700c)
Bridgestone MB series
Of course there are many others. These bikes were built tough and came with 36-spoke wheelsets, clearance for good size tires, and mounts for racks and fenders. The drivetrains are usually 3 x 7 (21 speed) but can easily be upgraded (for a price) but 7-speed parts are still readily available. The original seats and grips were atrocious but with a little hunting in the bargain bins at your LBS you should be able to find a decent modern seat for not too much money. For comfort and multiple grip positions, I'm a fan of Ergon grips but they'll run you about $40 a set. Regular MTB grips with bar ends work nearly as well at half the price. Replace the original knobby tires with some hybrid tires and you've got a commuting machine that is still at home on trails.
You should be able to pick one up in good working condition for <$200 which leaves you $100 for some creature comforts and accessories.
Stay away from the big box store bikes. I know some people seem to be happy with them, but the majority I've seen result in disappointment and end up sitting in a corner of the garage until they get sold at a yard sale or junked. I'll take my 1993 Trek 720 over anything that WalMart sells today.
Last edited by GravelMN; 05-06-15 at 11:40 PM.