There were a lot of related posts in Biostars.

Finding The Gene Annotation Output From Web Blast In The Xml Etc. ?

Gene Ontology Term Generation For Keywords

Getting Blastx Translated Query

A: From FASTA file to GO-annotation

A: which one is better to do, blastx or blastp?

Alternative way for annotation of assembled transcriptome using BLASTX

I would look through them plus NCBI search results to your question:

"to annotate the output of the blastx AND gene function AND gene ontology"

http://www.ncbi.nlm.nih.gov/pmc/?term=to+annotate+the+output+of+the+blastx+AND+gene+function+AND+gene+ontology

What is your target, the selection depends upon the organism you study and have sequenced?

You want to make it simple?

I've asked this:

"to annotate easily the output of blastx AND gene function AND gene ontology"

and I've got this:

http://www.ncbi.nlm.nih.gov/pmc/?term=to+annotate+easily+the+output+of+blastx+AND+gene+function+AND+gene+ontology

I've even found some soft for you.

"to annotate easily the output of blastx AND soft AND gene function AND gene ontology"

http://www.ncbi.nlm.nih.gov/pmc/?term=to+annotate+easily+the+output+of+blastx+AND+soft+AND+gene+function+AND+gene+ontology

Hopefully you will find some useful tool.