Investorwords.com defines a trademark as a “distinctive name, symbol, motto, or design that legally identifies a company or its products and services, and sometimes prevents others from using identical or similar marks.” There are many words in the English language that are, in fact, trademarks, but aren’t commonly known or recognized as such. This is a list of trademarked words, which should only be used when its certain that the specific brand or product is used or references. These words are not to be used a generic terms, and the generic terms are provided in each entry. This is not an exhaustive list, so if you are uncertain if a word could be a trademark, it’s best to look it up in either an AP Style Handbook or Wikipedia's list, which is a lot more comprehensive.
The only correct time to use a trademark is if you mean that particular brand of item (if someone is actually using Kleenex brand tissues, for example). But, if you mean generic facial tissues, then "facial tissues" needs to be said. Here is my list of some of the more commonly used trademarks, with the generic terms that ought to be used instead.
Band-Aid (bandage or adhesive)
ChapStick (lip balm)
Jacuzzi (hot tub or whirlpool)
Kleenex (facial tissue)
Onesie (infant bodysuit) - Yes, this is still trademarked by Gerber
Photoshop (Photo manipulation)
Ping Pong (table tennis)
Post-It (sticky note)
Q-Tips (cotton swabs)
Realtor (real estate agent) - The word actually refers to members of the National Association of Realtors, not real estate agents in general. The Association goes through great lengths to prevent the word from becoming generic.
Saran Wrap (plastic wrap)
Scotch Tape (clear adhesive tape, or simply tape)
Sharpie (permanent marker)
Speedo (swim briefs) - Yes, this one is also trademarked.
Vaseline (petroleum jelly)
Xerox (photocopier, or to make a copy)