High-Performance Linq-Like extension methods for arrays and List that avoid unnecessary allocations.
This is nowhere near ready for production, but I was recently reading about how Linq does allocations all over the places as well as multiple enumerations and that marshalling List through IEnumerable causes Enumerators that live in the stack to be pushed to the heap and thus cause more garbage collection. The solution is to use List whenever possible and T[] to avoid unnecessary heap allocations.
I also was thinking about typical use cases, such as Where().Select()[.Take()] chains that are so common and created new "combo" extension methods like SelectWhere, SelectTake, SelectWhereTake, etc that use a single loop to do all the work.
List<TData> data = GetFromSource<TData>();
var result = data.Where(d => d.Prop1 == 1).Select(d => (d.Prop1, d.Prop2)).Take(10).ToList();
// becomes
var result = data.SelectWhereTake(10, d => d.Prop1 == 1, d => (d.Prop1, d.Prop2));