If you grew more than one variety of tomato in your garden and found seedlings popping up in your garden or compost bin the following season, the tomato plants may or may not be natural F1 hybrids (the first generation after a cross). Tomatoes (having both female and male reproductive parts) usually self-pollinate and don’t cross-pollinate as easily as some other vegetables. Although, it is possible that cross-pollination occurred if the plants weren’t separated by 1/4 mile or more. If your tomatoes crossed, the result would be hard to predict. You might get some very tasty tomatoes with qualities of one or multiple varieties; you may get some fruits that are less desirable; or you may get perfect fruit with any off types not appearing unless you save the seeds from the volunteer plants and sow them the next season (producing an F2 generation). So, if you get some tomato volunteers, it’s up to you if you want to save them and grow them out. If you have the room and enjoy experimenting, you might vote for giving them a chance to produce just to enjoy the mystery.