I have been into freelance business for more than 2 years. While doing projects for other people, I've build a compilation of common tasks that I implement in projects and put them into code. It's kind of a library with some functions that I can reuse without having to rewrite the same thing dozen times. I'm talking about accessing Access databases, downloading information from FTP and similar stuff.

Is this acceptable from a legal point of view? What's the difference in reusing the old code and rewriting it from the scratch (using you own brain again, therefore the exact same logic)?

I do not hold any copyright to it, of course, and provide the source code for these classes to my clients.