Here are a few basic algorithms and data structures implemented in C. Most have some kind of documentation and tests, but I highly recommend you to proofread this with a copy of Cormen's Introduction to Algorithms or similar book.

• Vector
• Linked list
• Doubly linked list
• Stack
• Graph
• Trees
• Heap
• Binary tree
• Binary search tree (BST)
• AVL tree
• Red-black tree

• Sorting
• Heapsort
• Mergesort
• Quicksort
• Insertion sort
• Shortest path
• Dijkstra's shortest path

• Data structures
• Set
• Hash table
• B-tree (maybe...)
• Algorithms
• A* search