Most pre-2012 Huawei modems can be unlocked by IMEI for free here: http://www.mapmyplace.com/calc/hua.php
Do note that the Huawei E220 can’t be unlocked by code calculation and requires a cable/server option and this is not free.
New Huawei modems from 2012 use an encrypted algorithm and sadly can’t be unlock for free. The following site offers Huawei unlock codes for these new modems at £3.50 each which shouldn’t break the bank.
E173, E1732, E1815, E352, E353, E353Ws,E353Ws-2, E357, E362, E367, E368, E372, E392, E398, E589, E5832s and E589u-12 LTE
The site address: http://huawei.site90.com/
The site also has links to the latest version of unbranded Mobile Partner.