US7028023 seems to be a patent for a data structure (a variation on a linked list with an extra pointer to the item after the next item). Are data structures generally considered patentable, if so, are there other examples like this?
Depending on how it is claimed, probably yes. I realize it's a non-definite semi-answer, but the topic, as with all things patent-law, is complex and involves legal theories like "printed matter doctrine".
Very interesting discussion in this article (which also has a few examples):
Andrew Joseph Hollander, "Patenting Computer Data Structures: The Ghost, The Machine, and the Federal Circuit," Duke Law & Technology Review, December 2003
Data structures have been upheld as patentable when they have structural and functional relationships that provide a useful result. Data structures are not generally patentable if they are descriptive or recite a list. The manipulation of ideas is not patentable.
The claims do matter. Compare In re Lowry, 32 F.3d 1579 (Fed Cir 1994) (memory having a pyramidal data structure with structural and functional relationships to provide a useful result) with In re Warmerdam, 33 F.3d 1354 (Fed. Cir. 1994) (robotic collision avoidance description with a claimed method and a bubble data structure that were a mere algorithm and result).