By combining up to four microSD cards into a device called the MicroSD SSD Creator kit, you can create your own solid state disk from (old) memory cards. The Japanese company GeekStuff4U sells the device on their website for $79.56. The same company also lists another SSD that can be build from memory cards, the 2.5" DIY SSD. This device can contain up to three CompactFlash cards and is sold at $189.90.
The MicroSD SSD Creator kit measures 78x54x6 millimeters and weighs 13 grams. It can be used as a boot device for Windows XP, Windows Vista, Windows 7, Windows 8 and OSX. It's compatible with a SATA slot and supports four microSD cards with each a maximum capacity of 32 GB, allowing a total maximum capacity of 128 GB.
The CompactFlash based device fits in a 2.5" slot and allows for more capacity, you can fit in a maximum of three CF cards which combined can have a maximum capacity of 2 TB . It's available in either a SATA 1 or IDE Ultra DMA 33 version. There's also support for several CF interfaces.
Both the devices are limited in performance by the speed of the microSD or CompactFlash cards which is likely not competitive to actual SSDs.
