The Best of Dover Books (a.k.a the best cheap mathematical texts)

Perhaps this is a repeat question -- let me know if it is -- but I am interested in knowing the best of Dover mathematics books. The reason is because Dover books are very cheap and most other books are not: For example, while something like Needham's Visual Complex Analysis is a wonderful book, most copies of it are over $100.

In particular, I am interested in the best of both undergraduate and graduate-level Dover books. As an example, I particularly loved the Dover books Calculus of Variations by Gelfand & Fomin and Differential Topology by Guillemin & Pollack.

Thanks. (P.S., I am sort of in an 'intuition-appreciation' kick in my mathematical studies (e.g., Needham))

EDIT: Thank you so far. I'd just like to mention that the books need not be Dover, just excellent and affordable at the same time.

Solution 1:

Pinter's A Book of Abstract Algebra is a great introductory text!

Solution 2:

Though it lacks any treatment of cardinal functions, Stephen Willard’s General Topology remains one of the best treatments of point-set topology at the advanced undergraduate or beginning graduate level. Steen & Seebach, Counterexamples in Topology, is not a text, but it is a splendid reference; the title is self-explanatory.

Solution 3:

Nathan Jacobson's Basic Algebra I is pretty good, along with the sequel for the more brave of heart. (Disclaimer I haven't read II, but I imagine it is also good).