I went for a long time sticking mainly to O'Reilly. I bought books by other publishers but only when I couldn't find exactly what I was looking for with an O'Reilly book (which wasn't very often). The book which finally moved me off this was The Pragmatic Programmer (an awesome book which every programmer should read) which is published by Adisson Wesley. After that I started buying books from the Pragmatic Bookshelf (the version control and unit testing ones are brilliant), and then just started buying books which were getting a lot of attention in the PHP community (GOF, PoEAA, Matt Zandra's, the PHP|Arch. one etc.). What I think I've found is that different publishers seem to cater to certain areas of the market.
[WARNING]Broad generalizations and personal opinion comming up
Wrox: Entry level web books. Even their professional books tend to be very accessable.
O'Reilly: Reference material. I can't think of an O'Reilly book I've read cover to cover and I can't think of one which hasn't been usefull as a reference.
Addison Wesley: High level (in the programming sense of being more abstract rather than meaning more academic). Tend to be well laid out for reference, but a different kind to that of O'Reilly.
Pragmatic Bookshelf: Similar to Addison Wesley in a way but more focused around best practices rather than concrete development models (that one's probably going to get some comments 😉
Prentice Hall: Acedemic support. It's what they are, you can't really argue with that one.
APres: I've only got the one book by these guys I think (not sure, half my books are at the office). Matt Zandstra's OPP. Just had a look through what they're offering though and wow, looks like I might be doing another little spending spree soon.
Anyway, this is only touching on a few publishers. Those which I are or have been most relevent to me. Also, this is just my opinion and even in that they are broad generalizations. I have exceptions to these rules sat in front of me now. Programming Ruby (Addison Wesley) which would sit just as comfortably in an O'Reilly cover or Head First Design Patterns (O'Reilly) which, with the rest of the Head First series could carve their own publishing company in much the same way the Pragmatic Bookshelf did off the back of The Pragmatic Programmer.