You can use the UCSC Table Browser.

1. Change "group" to "Variation and Repeats"
2. Change the Track to"Common SNPS (135)"
3. Change the "Table" to "snp135common"
4. Click on identifiers: PASTE LIST and past in your rs numbers
5. Select "get all fields from selected table" as the output format
6. 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.