These things depend on location. Where are you? Here's how things work in SV.
Cover letter is exceptionally useful. It should explain in simple English why you are qualified for this particular job, highlighting relevant experience and showing that you actually bothered to figure out what this job/company is about. Doing this well is a huge advantage. At your level 1 out of 100 applicants will do it right. Cover letter should be just natural part of the body of the email and be in plain text. It should not be formal.
Suit with no tie is appropriate if you are interviewing for a job of CEO. Suit with a tie is never appropriate. This is different on east coast.
I'm in New York, where dress is a bit more formal due to the Wall Street sphere of influence. However, the position I have in mind has nothing to do with finance.
What defines a good and bad cover letter, in your opinion? What mistakes do the writers of the 99 bad cover letters make?
Although I can't say for sure (never hired anyone), I think the notes above about "Relevant experience" really hits it on the nail. Resume's have a lot of info on them, and are often short on details. You should explain to them what you've done that makes you ideal for that specific role. Don't talk about your life dreams, just keep it simple and direct.
On what to wear: Dress smart. If it seems like a fun place, dress a bit fun. In NYC, a nice pair of jeans, with a sharp shirt and jacket would probably be a really appropriate attire. I don't think i'd ever show up for an interview with just a t-shirt on, but i guess it depends.
Cover letter is exceptionally useful. It should explain in simple English why you are qualified for this particular job, highlighting relevant experience and showing that you actually bothered to figure out what this job/company is about. Doing this well is a huge advantage. At your level 1 out of 100 applicants will do it right. Cover letter should be just natural part of the body of the email and be in plain text. It should not be formal.
Suit with no tie is appropriate if you are interviewing for a job of CEO. Suit with a tie is never appropriate. This is different on east coast.