You can use the UCSC Table Browser.
- Change "group" to "Variation and Repeats"
- Change the Track to"Common SNPS (135)"
- Change the "Table" to "snp135common"
- Click on identifiers: PASTE LIST and past in your rs numbers
- Select "get all fields from selected table" as the output format
- Click "Get output"
If you need details of what is actually in the table, you can click on the "describe table schema" after step 3 above.
You can also get the MAF from the Biomart portal.
The following instructions are for Biomart version 0.7, because I still haven't learned how to use the newer version.
- Go to http://www.biomart.org/ and click on Version 0.7->Portal
- as database, select the latest Ensembl Variation dataset available, and as dataset, select the Human Variation (dbSNP) dataset, or the one for the organism of your interest
- In Filters, open GENERAL VARIATION FILTERS, select "Filter by Variation ID", and paste your list of SNPs
- In Attributes, open SEQUENCE VARIATION, and select Minor Allele Frequency and any annotation you may be interested into
- Click on Results to download the data.