Finding and Comparing Homologous Genes
There are various databases of homologs, for example:


eggNOG
Ensembl Compara (accessible via BioMart, I think)
Homologene
OMA
TreeFam


The advantage of using an existing database is that more sophisticated methods for detecting orthologs than simple BLAST searches have been used (see "Computational methods for Gene Orthology inference" for a review), and everything is already precomputed, so it's much faster. The downside is that all precomputed methods need to use a snapshot of the genomes from the past, so not all currently available genomes will be there.