Using PSSitecoreItemWebAPI

Add-SitecoreItem.ps1

# Adding as an authorised user is much the same as getting.  Just provide the template and name fields
Add-SitecoreItem sitecore.local -user apiuser -pass password -database master `
  -name "Awesome new item" -template "Sample/Sample Item"

<#
totalCount                   resultCount  items                                          StatusCode
----------                   -----------  -----                                          ----------
         1                             1  {@{Database=master; Displa...                         200
#>

# We can also say where we want the item created as well as set fields at the same time
Add-SitecoreItem sitecore.local -user apiuser -pass password -database master `
  -path /sitecore/content/home -name "Awesome new item" -template "Sample/Sample Item" `
  -itemFields @{title="This is a title"; text="<h1>Rich text ahoy!</h1>"}

<#
totalCount                   resultCount  items                                          StatusCode
----------                   -----------  -----                                          ----------
         1                             1  {@{Database=master; Displa...                         200
#>

# Use "Get-Help Add-SitecoreItem" to see what else you can do

Get-SitecoreItem.ps1

# As an anoymous user
Get-SitecoreItem sitecore.local | select -expand items

<#
Database    : web
DisplayName : Home
HasChildren : False
ID          : {110D559F-DEA5-42EA-9C1C-8A5DF7E70EF9}
Language    : en
LongID      : /{11111111-1111-1111-1111-111111111111}/{0DE95AE4-41AB-4D01-9EB0-67441B7C2450}/{110D559F-DEA5-42EA-9C1C-8A5DF7E70EF9}
Path        : /sitecore/content/Home
Template    : Sample/Sample Item
Version     : 1
Fields      : @{{75577384-3C97-45DA-A847-81B00500E250}=; {A60ACD61-A6DB-4182-8329-C957982CEC74}=}
#>

# As an authorised user getting content from a particular database
Get-SitecoreItem sitecore.local -user apiuser -pass password -database master | select -expand items

<#
Database    : master
DisplayName : Home
HasChildren : False
ID          : {110D559F-DEA5-42EA-9C1C-8A5DF7E70EF9}
Language    : en
LongID      : /{11111111-1111-1111-1111-111111111111}/{0DE95AE4-41AB-4D01-9EB0-67441B7C2450}/{110D559F-DEA5-42EA-9C1C-8A5DF7E70EF9}
Path        : /sitecore/content/Home
Template    : Sample/Sample Item
Version     : 1
Fields      : @{{75577384-3C97-45DA-A847-81B00500E250}=; {A60ACD61-A6DB-4182-8329-C957982CEC74}=}
#>

# You can also provide -path / -query / -item / -scope and many more to filter items
# Use "Get-Help Get-SitecoreItem" to see what is available to you

Remove-SitecoreItem.ps1

# Removing an item is as easy as you'd expect
Remove-SitecoreItem sitecore.local -user apiuser -pass password -database master `
  -item "{544F86D9-B0B2-497F-8E73-61531708CEFA}"

<#
totalCount                   resultCount items                                            StatusCode
----------                   ----------- -----                                            ----------
         1                             1 {@{ID={544F86D9-B0B2-497F-...                           200
#>

# Note: the api only returns the ID of the items removed

Set-SitecoreItem.ps1

# Setting properties on an item is much the same as adding an item.  This time the name and template are not needed
# Note: here we're using the -item parameter to ensure we are setting on this specific item
Set-SitecoreItem sitecore.local -user apiuser -pass password -database master `
  -item "{544F86D9-B0B2-497F-8E73-61531708CEFA}" `
  -itemFields @{ Text = "<strong>updated</strong> with something <em>else</em>"}

<#
totalCount                   resultCount items                                            StatusCode
----------                   ----------- -----                                            ----------
         1                             1 {@{Database=master; Displa...                           200
#>

# Again you can get fancy here and use -query / -scope etc to update multiple items in one go!
# Use "Get-Help Set-SitecoreItem" to see what is available to you