Lots of good answers here. NaCl (salt) is one (relatively) recent effort to be just such a library, see eg under the sub-heading "High Level Primitives" on the features page:

  http://nacl.cr.yp.to/features.html
 
  High-level primitives

  A typical cryptographic library requires several
  steps to authenticate and encrypt a message.
  Consider, for example, the following typical
  combination of RSA, AES, etc.:

  * Generate a random AES key.
  * Use the AES key to encrypt the message.
  * Hash the encrypted message using SHA-256.
  * Read the sender's RSA secret key from
    "wire format."
  * Use the sender's RSA secret key to sign the
    hash.
  * Read the recipient's RSA public key from wire format.
  * Use the recipient's public key to encrypt the
    AES key, hash, and signature.
  * Convert the encrypted key, hash, and signature to wire
    format.
  * Concatenate with the encrypted message. 

  Sometimes even more steps are required for storage
  allocation, error handling, etc.

  NaCl provides a simple crypto_box function that
  does everything in one step. The function takes the
  sender's secret key, the recipient's public
  key, and a message, and produces an authenticated
  ciphertext. All objects are represented in wire
  format, as sequences of bytes suitable for
  transmission; the crypto_box function
  automatically handles all necessary conversions,
  initializations, etc.

Of course, "such libs have bugs" -- it is software after all. But bugs can (and will be) fixed.

Somewhat unique to security and cryptography are the number of subtle bugs possible. There are both problems of actual "normal" bugs (like the Debian entropy bug) and system level design errors (like CRIME).

NaCl/Salt tries to reduce the number of errors possible by using the library wrong (as opposed to eg: openssl that has a very (some say too) rich interface). But you could still end up writing the secret key to swap. Or doing something silly with the plain text. Or expose yourself to a buffer overflow in the part of the code that renders those cute avatar-images for your chat application.

edit: formating