Link2SD is an application that makes it easy for Android 1.6+ users with root privileges on their phone to move applications to the SD card. 

The idea behind is same, linking to SD card's second partition but differs in two ways.

One, instead of linking /data/app folder (that means moving all applications), you can select applications that you want to move. 

Two, it's an application which means has a user interface. You can manage your applications, create or remove links with just one click. 

It also provides native apps2sd features on Android 2.2 and higher. Plus, it can move any non-protected user apps to SD card (force move), with batch moving capability.
That feature uses Android's standard SD-card installation feature and does not require a second partition. It can break some apps if they are not meant to be moved to the SD card (Such as Widgets).

Features:

• Create and remove link for apk, dex and lib files of the applications you select

• Automatically link newly installed apps

• Move any user apps to SD even though the app does not support moving to SD (for Android 2.2 and higher)

• Set the default install location of the apps; auto, internal, or external (for Android 2.2 and higher)

• Batch link, unlink, reinstall, uninstall, "move to SD", "move to phone" functions (multi-select mode)

• Uninstall system applications

• Freeze and un-freeze system and users applications

• Convert system apps into user apps

• Convert user apps into system apps

• Integrate "Updated" system apps into system (ROM)

• Clear data and cache of the application

• List applications, show detailed size information and link status

• Sort and filter applications

• Search applications by name

• Display available space information of internal storage, SD card and SD card 2nd partition

What you need for linking applications:

• root permission.

• a second partition on your SD card. You should have two partitions on SD card and both should be primary.
  
  The first FAT partition is your standard SD card storage. The second partition is used for application files and can be ext2, ext3, ext4 or FAT32.
  ext4 is the latest ext format but several ROMs have issues with it while fewer have issues with ext3 and ext2. FAT works on all ROMs.
  
  I use free "MiniTool Partition Wizard Home Edition" (windows application) for this.
  You can create the second partition without repartitioning, formatting or deleting your existing partition with this tool. Connect your SD card to PC with a card reader or phone itself and make it visible as a drive. Open Minitool select this drive and resize down FAT partition, you will see an unallocated area at the end of the card. Create ext2, ext3 or FAT32 partition in that area (create as primary, not logical), click apply. Unplug from the PC and reboot the phone. (To be on the safe side backup your data before starting)

• That's all. Just install Link2SD, it will do all necessary. No need to copy files or run scripts.*

* Some phones lack the ext2, ext3 or ext4 file system support and you may get mount errors like "no such device", "invalid argument". These phones need a matching ext2.ko or ext3.ko modules and even some additional modules to be able to mount. 
Using FAT32 on the second partition will eleminate such problems, because FAT32 is supported on all phones.

How it works:
Link2SD mounts the second partition of your SD card as /data/sdext2 and makes OS mounts it at boot time. 

When you select an application and press "Create Link" button you can select which files of the application will be linked and moved to SD card. Apk, dex and lib files can be moved and you can select all three or any two of three or even only one of them.
Depending on what you select, Link2SD moves apk file, dalvik-cache (.dex) file and lib files (.so) to SD card's second partition and creates symbolic links in original directories.

Note: On Android 1.6 and 2.1, shortcut and the widget of the app should be re-added to homescreen if you move the apk file.

Can I remove the links and put application files back to phone?
Yes, just click on application that is linked and press "Remove Link" button. You can select which files of the application will be unlinked and moved back to internal storage.
Then, Link2SD removes the symbolic links and moves the files you selected to original directories.

What happens if I uninstall or update a linked application?
If you uninstall a linked application apk, dex and lib files will be automatically deleted from the SD card by Link2SD.
If you update a linked application new apk, dex and lib files will be placed in the internal memory at first and then they will be automatically relinked by Link2SD in the background.
You don't have to manually relink after updating applications.