NPOI

Advantage of NPOI

a. It's totally free to use

b. Cover most features of Excel (cell style, data format, formula and so on)

c. Professional support service (even 24*7) from NPOI team (not free)

d. Support xls, xlsx, docx.

e. Designed to be interface-oriented (take a look at NPOI.SS namespace)

f. Support not only export but also import

g. .Net 2.0 based even for xlsx and docx (though we also support .NET 4.0)

h. Successful cases from all over the world

i. huge amout of basic examples

Ref:http://npoi.codeplex.com/