Besides trademarks, name choosers need to worry about Common Law rights. Common Law rights are rights that are accrued through use and are powerful, even if the user has never registered their name as a trademark, or acquired the URL. Common law rights to names used by others can prevent your name use for similar goods, so you need to know who else is out there, where, and what they are selling or otherwise delivering.
While a somewhat grey area, you need to consider how close their name is to yours, and how close their goods or services are to yours; if too close and you go to market with your name, they could sue you and really slow you down. The key issue for someone to win a trademark case against you is to prove the likelihood of consumer confusion, or that you chose your name merely to take advantage of them.
If you think someone already using your name could successfully assert either of these complaints, your would be wiser to choose a different name.