BioMart has orthologs - you can enter a list of AtG IDs into the Ensembl Plants section and retrieve orthologs in other plant species.

Thanks for the idea Jonathan, is there any advantage to downloading the BioMart software or can all its functions be accessed through the webbrouser? However, i'm more interested in looking for paralogues...

You can get the data from the website without needing the software.

You might find it helpful to use a normal BLAST and then click 'Distance Tree of Results' or 'Taxonomy reports'. These will allow you to see which results are from species that are taxonomically close to Arabidopsis.

These databases have some identified gene families.

http://bioinformatics.psb.ugent.be/plaza/

http://genomics.msu.edu/cgi-bin/plant_specific/family_search.cgi

http://www.phytozome.net/

http://www.greenphyl.org/cgi-bin/index.cgi

You can build your own pipeline to identify orthologs (e.g. orthoMCL) and construct trees (many softwares).